Determining the index and optimal irrigation depths to maximize benefit of early maturing corn

To calculate the index and optimal depths of crop water use, and maximizing the profit under deficit irrigation practices, this study was performed using a split plot design. In this experiment three cultivars of early maturing corns (301, 303 and 315) were set as the main treatments with 3 replications, while the irrigation levels were considered to be the minor treatments. A line source irrigation method (after Hanks) with 6 irrigation levels on both sides of the line was used. In this research, three sub-functions such as yield Y(w), cost C(w), and benefit B(w) were developed initially. Then, the optimal depths of irrigation were extracted from these functions. The results indicated that: a) although the complete irrigation had the maximum yield, however, the marginal net profit was not maximum because of the cost rise, b) with deficit irrigation under land restriction conditions and aiming to maximize the use of unit land, the optimized water depth for all cultivars was 3% less than the complete irrigation, and c) using deficit irrigation under water limitations and aiming to maximize the use of unit volume of water, the optimized irrigation depth would be 19% less than the complete irrigation practices. Also, the results showed that the net benefit was the same for the equivalent depth and the maximum water depth; therefore, it is logical to use the equivalent depth. Meanwhile, applying 17.5% deficit irrigation for cultivar 301, 20.4% for cultivar 303 and 19.1% for cultivar 315, the highest earning return (Rials per m3 of water) will be 1011.2, 1206.1 and 1543.6 Rials, respectively. It can be concluded that with the savings of water under deficit irrigation practices, the planting area would increase and ultimately the marginal net profit increases substantially. Also, the most net profit among the cultivars was obtained for cultivar 315.